1. Field of the Invention
The field of the present invention generally relates to hydrostatic transmissions and transaxles and, more particularly, is concerned with the hydraulic fluid system and disengagement mechanism.
2. Description of the Prior Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, then back to rotary mechanical motion to rotate a drive axle in order to drive a vehicle, such as a lawn and garden tractor. The hydrostatic transmission regulates or controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Within a hydrostatic transmission of the radial piston type, a cylinder unit having a pump and motor, rotates on a fixed pintle with pistons positioned within the cylinders and attached to slippers mounted in an expander band so that as the cylinder unit rotates, the pistons are driven by the slippers which engage the surrounding eccentric annular track ring of the pump and motor. In a swash-plate type hydrostatic transmission, the pistons reciprocate in a parallel fashion. The pistons of the pump create a pressurized fluid flow that drives the motor pistons which rotate an output shaft. The transmission ratio is therefore directly proportional to the eccentricity of the track ring or swash-plate of the pump. The radial piston type hydrostatic transmission is shown in U.S. Pat. No. 4,979,583 entitled Variable Speed Transaxle, issued Dec. 25, 1990, and pending application Ser. No. 07/535,462 entitled Variable Speed Transaxle filed Jun. 8, 1990 both of which are specifically incorporated by reference herein.
One common application of a hydrostatic transmission is within a riding lawn mower. Although lawn mowers are self-propelled, in many situations such as positioning the mower in a garage or because of engine failure, it is necessary to manually push the mower. Hydraulic transmissions however, present a special problem when the vehicle is to be manually moved. With gear transmissions a neutral mode or position is achieved by mechanically disengaging the input shaft from the output shaft by either disengaging the gears coupling the input shaft to the output shaft or disengaging the output shaft from the gears. Thus it is possible to manually move the vehicle without resistance from the transmission system either from the output shaft or the input shaft since they are disengaged from one another.
Unlike gear transmissions, in hydraulic transmissions the hydraulic motor forming part of the hydrostatic unit and its output remains hydraulically engaged with the pump even when in a neutral position thereby presenting substantial resistance to movement. Thus, when trying to push a vehicle having a hydrostatic transmission, even when in neutral, the output acts as the input, and consequently the motor will attempt to pump hydraulic fluid back toward the pump. It is thus advantageous to provide a hydrostatic transmission with a disengaging mechanism that allows the unit to be more easily moved.
Conventional hydrostatic transmissions provide disengagement by opening check valves within the pintle, for example, of the hydrostatic transmission thereby short circuiting the oil passages which connect the hydraulic pump and motor so the oil will not be pumped back into the pump. The short circuit in the oil passages prevents the rotation of either the input or output shafts from effecting the movement of the other. The disengaging mechanism is often termed a relief valve or dump valve since it relieves or "dumps" the hydraulic fluid from the pintle or fluid conduit connecting the pump and the motor into the chamber surrounding the hydrostatic transmission within the hydrostatic transaxle housing.
Dump valves of the prior art however, have involved utilizing complicated and unwieldy control mechanisms within the limited area inside the housing of the transmission, adding to the expense while decreasing the efficiency and reliability of the hydrostatic transmission.
In addition, oil that circulates through the transaxle housing must be free from particulates or foreign matter before reentering the pintle, as the pistons of the pump and motor can be damaged during operation by particulates suspended in the oil as the pistons reciprocate.
Also, oil utilized in the gearing chamber of the hydrostatic transaxle should preferably not mix with the oil utilized in the transmission chamber because the gearcase oil will carry dirt and metal filings from the gears, which can cause damage to the hydrostatic transmission components. The transmission chamber and the gearing chamber are in communication via an output drive shaft that operably connects the transmission motor with the gearing that rotates the axle, thus is it necessary to provide sealing between the hydrostatic chamber and the gearing chamber in order to prevent leakage of the gear chamber oil into the hydrostatic chamber.
A prior art solution to prevent leakage is to utilize a single lip seal on the output shaft, but this tends to cause migration of the oil at shutdown when there is a lower pressure in the hydrostatic chamber than in the gearing chamber. Another solution is to provide two separate seals on the output shaft, although this tends to cause an undesirable vacuum between the two seals.